if the lake were full of trout, which might increase property value if uncontaminated and fish-able, a word that implies an increase in property value could be "brimming" or "flush" (e.g. "I am flush with ocelots" -3rd rock from the sun, beanie babies episode)

While the word 'infested', which was obviously ill-chosen, does convey notions of contamination et al, the eel-like sea lamprey could someday enable paralyzed people to reliably control their legs.

You see, the creature has a removable spine that can remain alive in a dish and be stimulated to move as if it's still inside a swimming animal. It is possible to transplant the spinal chord into a human being and control the signals via outside controls, thereby enabling the paralyzed individual to move.

This is a veritable medical miracle which must be worth millions. It is obvious to me that the lake containing these wonderous creatures would easily be considered the more valuable of the two.